Exhaust aftertreatment systems are used to receive and treat exhaust gas generated by IC engines. Generally exhaust gas aftertreatment systems include any of several different components to reduce the levels of harmful exhaust emissions present in exhaust gas. For example, certain exhaust gas aftertreatment systems for diesel-powered IC engines include a selective catalytic reduction (SCR) system, including a catalyst formulated to convert nitrogen oxide (NOx) gases (nitric oxide (NO) and nitrogen dioxide (NO2) in some fraction) into harmless nitrogen gas (N2) and water vapor (H2O) in the presence of ammonia (NH3). Generally in such aftertreatment systems, an exhaust reductant (e.g., a diesel exhaust fluid such as urea) is injected into the SCR system to provide a source of NH3 and is mixed with the exhaust gas to partially reduce the NOx gases. The reduction byproducts of the exhaust gas are then fluidically communicated to the catalyst included in the SCR system to decompose substantially all of the NOx gases into relatively harmless byproducts that are expelled out of the aftertreatment system.
SCR systems generally demonstrate a low NOx conversion efficiency (e.g., less than 90% NOx conversion efficiency) when the exhaust gas passing through the SCR system is at a relatively low temperature (e.g., less than 200 degrees Celsius). For example, when an engine is cold started, it may take significant time before the temperature rises enough that the SCR system is able to provide a high NOx conversion efficiency. Furthermore, the exhaust gas may cool as it expands in a turbocharger, so it may not achieve or maintain a temperature sufficient to achieve or maintain a high NOx conversion efficiency.